Units: M

Units: M

M [1]
an informal abbreviation for million in expressions such as “$500M” for
500 million dollars or “Unemployment Reaches 4M” in a newspaper
headline. In binary contexts such as computer memory, M often represents 220
= 1 048 576 (see mebi-, below).
M [2]
the Roman numeral 1000, sometimes used in symbols
to indicate a thousand, as in Mcf, a traditional symbol for 1000 cubic
feet. Given the widespread use of M to mean one million, this older use of
M to mean one thousand is very confusing and should be scrapped.
M [3]
the symbol for “molar” in chemistry (see below).
a symbol for one million years, often used in astronomy and geology. The
“a” stands for the Latin annum.
symbol for “meters above bottom” (bottom of the sea), a unit used in oceanography.
a traditional Chinese unit for weighing precious metals, especially silver.
In the European colonial period, the mace was considered equal to 0.1 tael
or liang; this would be 2/15 ounce
or about 3.78 grams.
Mach or mach (M or Ma)
a measure of relative velocity, used to express the speed of an aircraft
relative to the speed of sound. The name of the unit is often placed before
the measurement. Thus “Mach 1.0” is the speed of sound, “Mach 2.0” is twice
the speed of sound, and so on. (The actual speed of sound varies, depending
on the density and temperature of the atmosphere. At 0 °C and a pressure
of 1 atmosphere the speed of sound is
about 1088 ft/s, 331.6 m/s, or 741.8 mi/h). The mach speed is important to
the control of an aircraft, especially at speeds close to or exceeding Mach
1.0. The unit is named for the Austrian physicist Ernst Mach (1838-1916).
maf or Maf
a symbol for one million acre feet. This
symbol, commonly used in reservoir management in the U.S., should be written
Maf. 1 Maf = about 1.2335 billion (109) cubic meters.
magnitude (mag) [1]
a unit traditionally used in astronomy to express the apparent brightness
of stars, planets, and other objects in the sky. For centuries, the brightest
stars were said to be of the “first magnitude,” with fainter ones of the “second
magnitude” and so on down to “sixth magnitude” for the faintest stars visible
to the unaided eye. When it became possible to measure stellar brightnesses
precisely, it was discovered that stars of a given traditional magnitude were
roughly 2.5 times brighter than stars of the next magnitude. Astronomers agreed
to define the magnitude scale so that a difference of exactly 5.0 mag corresponds
to a brightness difference of exactly 100 times. A difference of 1.0 mag then
corresponds to a brightness difference of the fifth root of 100 or about 2.512
times. The scale is upside down: brighter stars have lower, not higher magnitudes,
in keeping with the historical origin of the scale. The zero point (0.0 mag)
is set arbitrarily so that the stars historically listed as “first magnitude”
have magnitude measurements of 1.5 mag or brighter. The brightest stars and
planets have negative magnitudes on this scale. Note: the scale is commonly used to describe the apparent magnitude of objects as we view them on Earth, but astronomers also use it for absolute magnitude, which is the magnitude the object would have if it were placed at a standard distance of 10 parsecs (32.61 light years) from earth.
magnitude (mag) [2]
a unit used in earth science to measure the intensity of earthquakes. Geologists
actually use several scales to measure earthquake intensity, but the one best
known to the public is the Richter magnitude scale, developed in 1935 by Charles
F. Richter (1900-1985) of the California Institute of Technology. The Richter
magnitude is computed from the measured amplitude and frequency of the earthquake’s
shock waves received by a seismograph, adjusted to account for the distance
between the observing station and the epicenter of the earthquake. An increase
of 1.0 in the Richter magnitude corresponds to an increase of 10 times in
the amplitude of the waves and to an increase of about 31 times more energy
released by the quake. The most powerful earthquakes recorded so far had magnitudes
of about 8.5. The Richter magnitude measures the intensity of the earthquake
itself, not the intensity of the earthquake’s effects: the effects also depend
on the depth of the earthquake, the geology of the area around the epicenter,
and many other factors. Earthquake effects are rated using the Mercalli
(see below).
a traditional unit of volume for wine, generally equal to 2 bottles.
This is now exactly 1.5 liters (about 2.114 U.S. quarts).
a traditional Arab weight unit equal to about 2.04 pounds or 925 grams.
a Norwegian word for “measure,” mål has been used as a name for various
traditional Norwegian units. As a land measure, the mål is currently
defined to be the same as the dekare, that is, exactly 1000 square meters
(0.1 hectare or 0.2471 acre).
The mål has also been used as a unit of volume equal to the dekaliter
(10 liters).
a traditional German unit of quantity equal to 15.
man hour
a common unit of labor equal to the work of one person for one hour. The
name person hour is increasingly used for this unit.
an informal unit of power equal to 0.1 horsepower
or about 74.57 watts. The unit seems to have
been invented by American engineers.
a traditional unit of land area in Central America. The manzana is the area
of a square 100 varas on a side; it thus varies
according to the length of the vara. The Costa Rican manzana equals 0.698
896 hectares or about 1.727 acres. Very similar
units are used in Guatemala, Honduras, and Nicaragua. The word manzana
means an apple, but the unit is probably related to manzanar, orchard.
a traditional unit of distance used in athletics. The length of a marathon
is exactly 42 195 meters (about half an inch longer than 26 miles 385 yards).
Invented for the first modern Olympic Games at Athens in 1896, the marathon
recalls a run made in 490 BC by a Greek soldier (possibly Pheidippides) to
bring to Athens the news of the Greek victory over the Persians at the Battle
of Marathon. However, the actual distance from Marathon to Athens is only
about 36.75 kilometers. The 1896 run was exactly 40 kilometers from the Marathon
Bridge to the Olympic Stadium. At the 1908 Olympics in London, a course of
26 miles 385 yards brought runners from Windsor Castle to White City Stadium
(the story is that exactly 26 miles was intended, but Queen Alexandra insisted
that the finish line be moved in front of the royal box). The marathons at
the Olympic Games varied in length until the 1924 Olympics in Paris, when
the International Olympic Committee adopted the 1908 London distance as official.
marc, marco, or mark
traditional units of weight in various countries of Western Europe. In each
country the unit equals 1/2 the unit corresponding to the English pound.
Thus the French marc equals 1/2 livre,
8 onces or about 244.75 grams; the Spanish marco equals 1/2 libra
or about 230 grams; the German mark equals 1/2 pfund or about 280.5 grams;
and the English mark equals 8 ounces or 226.8 grams. The English unit was
used almost entirely for measuring precious metals.
marine league
an informal name for the league as used
at sea: a unit of distance generally equal to 3 nautical
(5556 meters).
mark twain
see twain.
a traditional unit of area in Pakistan. The marla was standardized under
British rule to be equal to the square rod [1],
that is, 272.25 square feet, 30.25 square yards, or 25.2929 square meters.
symbol for milliarcsecond, a unit of
angular measure commonly used in astronomy.
a traditional unit of mass in India and Pakistan, standardized under British
rule as 15 grains, 1/12 tola,
or about 0.972 gram. In Pakistan, the unit is still used sometimes for the
weight of precious metals.
a common symbol for “meters above sea level” used in geology and geography.
maß (mass)
a unit of volume for beer in Germany and Austria, usually equal to one liter
today. The traditional Bavarian maß was about 1.07 liters.
a symbol for the milli-absorbance unit.
An increase in absorbance of 1 mAU corresponds to a reduction in transmittance
of about 0.2305%.
a traditional unit of weight in India and throughout South Asia. The maund
varied considerably, but during the period of British rule in India it was
standardized at about 82.286 pounds or 37.3242
kilograms. The maund is divided into 40 seers.
Since 1980, Pakistan has used a metric version of the maund equal to exactly
40 kilograms (88.185 pounds), thus making the seer equal to the kilogram.
maxwell (Mx)
a CGS unit of magnetic flux, equal to 10-8weber. In a magnetic field of strength one
gauss, one maxwell is the total flux across
a surface of one square centimeter perpendicular to the field. This unit was
formerly called the line [2]. The newer
name honors the British physicist James Clerk Maxwell (1831-1879), who presented
the unified theory of electromagnetism in 1864.
traditional symbols for 1000 (not one million) board
, a unit of volume for timber equal to 250/3 = 83.333 cubic feet or
2.360 cubic meters. “BM” stands for “board measure.”
a common symbol for “meters below sea level” used in geology and oceanography.
MBH, MBtuh
symbols for 1000 (not one million) Btu (British
thermal units) per hour, a unit traditionally used in the U.S. heating and
air conditioning industry to state rates of heating or cooling. One MBH equals
about 0.293 071 kilowatt.
mc- or mc [1]
alternate symbol for micro- (see below). This prefix is often seen in the
symbol mcg for the microgram. The use of the symbol mc- for micro-
became established because typewriters and early dot matrix printers did not
have the proper symbol µ-. Also, many hospitals require the use of mc-
in all handwritten notes and records, because a hastily written “µ”
may be mistaken for an “m,” leading to serious dosing errors. However,
the use of mc- for micro- is also confusing and should be avoided as much
as possible. One source of confusion is that mc- may be interpreted as “millicenti-“,
a mistake that could also lead to dangerous dosing errors. Occasionally, mc
has been used as a symbol for the micron (see below).
mc [2]
Italian abbreviation for the cubic meter (metro cubico). This is
a non-standard symbol; the proper symbol is m3.
a traditional symbol for 1000 (not one million) cubic feet, a unit of volume
equal to about 28.317 cubic meters.
a symbol for 1000 (not one million) cubic feet per day, a unit of water
flow used by many U.S. water supply companies and agencies. 1 Mcfd = 19.665
liters (5.195 U.S. gallons) per minute.
a symbol used in the natural gas industry for 1000 (not one million) cubic
feet of gas equivalent (cfe). This is really
an energy unit equal to about 1.091 gigajoules
an alternate symbol for the microgram. Although the SI symbol µg is preferable
in print, the symbol mcg is used widely in medicine, and its use is required
in many hospitals and clinics. This is because a handwritten µg is too easily
mistaken for the milligram symbol mg, possibly leading to serious medical
a symbol sometimes used for the microliter (µL).
a symbol for 1000 circular mils, a
unit of area equal to about 0.5067 square millimeter commonly used in
stating wire gauges. This symbol is being replaced by the less-confusing symbol
a symbol used for 1000 (not one million) centipoises, a unit of dynamic viscosity. This is a jarring addition of an obsolete English prefix to a metric unit, and its use risks a major misunderstanding since M- is the metric prefix for a million rather than a thousand. 1 Mcps equals 10 poises, so the proper name of the unit is decapoise (daP).
a symbol for milk clotting unit, used for measuring dosage of bromelain,
an enzyme used as a digestive aid and for reduction of pain and inflammation.
This unit cannot be converted to a weight unit, because different preparations
of the enzyme differ in activity. Bromelain is also measured in gelatin digesting
units (GDU); 1 MCU equals approximately 2/3 GDU.
a unit of quantity formerly used by fishermen. The mease equals the number
of herring in a basket, roughly 620.
a musical unit representing a series of beats
[2] (rhythmic stresses) with one primary or accented stress. A measure is
also called a bar because the end of a measure is represented in musical
notation by a vertical bar.
measurement ton (MTON or MT)
a unit of volume used for measuring the cargo of a ship, truck, train, or
other freight carrier, equal to exactly 40 cubic feet, or approximately 1.1326
cubic meters. This unit was traditionally called a freight ton (see ton
[5]), but that term now means a metric ton of freight in most international
usage. However, the confusion seems impossible to dispel; some shippers are
now using “measurement ton” to mean a metric ton of freight. (The way out
of this dilemma is simple: measure volume in cubic meters and weight in metric
mebi- (Mi-)
a binary prefix meaning 220 = 1 048 576. This prefix, adopted
by the International Electrotechnical Commission in 1998, is intended to replace
mega- for binary applications in computer science. (This replacement does
not seem to be happening.) The prefix is a contraction of “megabinary.”
a common symbol for “minimum erythemal dose,” the smallest amount of ultraviolet
radiation that produces observable reddening (erythema) of the skin. (Skin
is sensitive to reddening by radiation in only a narrow band of wavelengths
around 300 nanometers.) The MED obviously varies from one person to another.
Doctors and tanning salon operators typically use a value of 200 joules
per square meter (J/m2), which represents the MED of a highly sensitive
individual; persons with dark skin have MED’s in the range of 1000 J/m2.
Regulatory agencies are moving to use of the standard erythemal dose (SED),
a unit equal to exactly 100 J/m2. In tanning, a dose rate of one
MED per hour is equivalent to 55.55 milliwatts
per square meter of skin surface.
informal contraction of “megabyte,” used in computer science.
mega- (M-) [1]
a metric prefix meaning 106, or one million. (The form meg- is
often used before a vowel, as in megohm for one million ohms.) The
prefix is also common in ordinary language, meaning “very large,” as in megabucks
or megadose. The prefix is derived from the Greek word for large, megas.
mega- (M-) [2]
in measuring the storage capacity of a computer, the prefix mega- often
means 220 = 1 048 576 instead of an even one million. By a 1998
resolution of the International Electrotechnical Commission, the new prefix
mebi- (Mi-) is supposed to replace mega- for 220.
megabar (Mbar)
a metric unit of pressure. The megabar equals one million bars,
100 gigapascals (GPa) or about 14.503 million
pounds per square inch. Such intense pressures are found inside the earth
or in various advanced scientific experiments.
megabarrel (Mbbl, Mbo, MMb, or Mb)
a unit of volume used in the energy industry, equal to one million barrels
of oil. One megabarrel equals 42 million U. S. gallons,
which is about 158.987 megaliters (ML).
megabase (Mb)
a unit of genetic information equal to the information carried by 1 million
pairs of the base units in the double-helix of DNA; also used as a unit of
relative distance equal to the length of a strand of DNA containing 1 million
base pairs. In humans, one megabase corresponds approximately to a gene separation
of one centimorgan.
megabecquerel (MBq)
a unit of radioactivity equal to one million atomic disintegrations per
second or 27.027 microcuries.
megabyte (MB)
this unit of information is very common in the computer world, but it is
poorly defined. Often it means 1 000 000 bytes, but sometimes it means 220
= 1 048 576 bytes. As if that weren’t confusing enough, the 1.44 megabytes
stored on “high density” floppy disks are actually megabytes of 1 024 000
bytes each. This uncertainty is a major reason for the recent decision of
the International Electrotechnical Commission to establish new binary
for computer science.
megacycle (Mc)
1 million cycles, a term sometimes used as an informal name for the megahertz.
megacycle per second (Mc/s)
an older name for the megahertz.
megadalton (MDa)
a unit of mass equal to one million atomic mass
. See dalton.
megaflops (Mflops)
a unit of computing power equal to one million floating point operations
per second. See flops.
megagram (Mg)
an SI unit of mass equal to one million grams or 1000 kg. This means the
megagram is identical to the tonne (metric
ton). Large masses are almost always stated in tonnes in commercial applications,
but megagrams are often used in scientific contexts. One megagram equals about
2204.623 pounds.
megahertz (MHz)
a common unit of frequency equal to one million per second. Frequencies
of radio waves are commonly stated in megahertz.
megajoule (MJ)
a common metric unit of work or energy. The megajoule equals one million
joules, which is approximately 737 562 foot pounds, 947.8170 Btu,
238.846 (kilogram) Calories, or 0.277 778
kilowatt hours.
megakelvin (MK)
a unit of temperature equal to one million kelvins. This unit is used in
astrophysics; temperatures in megakelvins are found in the interiors of stars
or in highly excited plasmas. The reciprocal
(MK-1) is used in colorimetry.
a CGS unit of energy equal to 106ergs
or 0.1 joule (0.073 756 foot pound). The “l”
was added to “mega-erg” to make the unit pronounceable.
a metric unit of magnetic flux, equal to one million lines
[2] or 0.01 weber.
megaliter (Ml or ML)
a metric unit of volume equal to 1000 cubic meters. Commonly used in reservoir
and water system management outside the U.S., the megaliter equals 264 172
U.S. gallons or 0.810 713 acre
megalithic yard
a unit of distance equal to about 83 centimeters or 2.72 feet, defined in
1951 by the Scottish engineer Alexander Thom (1894-1985). Thom claimed this
unit was used in the construction of many megalithic monuments, including
Stonehenge. If so, the unit was probably measured by the length of a workman’s
megameter (Mm)
a metric unit of distance equal to 1000 kilometers or about 621.371 miles.
Although this appears to be an appropriate unit for longer distances on the
earth, the megameter is seldom used.
megampere (MA)
a unit of electric current equal to one million amperes.
This unit is used in plasma physics and fusion research.
meganewton (MN)
a metric unit of force equal to one million newtons. One meganewton equals
about 101 972 kilograms of force or 224 809 pounds
of force
. The main engines of the U.S. space shuttle have a maximum thrust
of about 2.28 meganewtons.
megaohm (megohm)
a common unit of electric resistance equal to one million ohms.
The spelling megohm is also used.
megaohm (megohm) centimeter
a unit of resistivity, used for pure water and for other substances having
relatively high resistivity. In the case of water, resistivity is a measure
of purity: the higher the purity, the higher the resistivity. The resistivity
of a conductor in megaohm centimeters is defined to be its resistance (in
megaohms) multiplied by its cross-sectional area (in square centimeters) divided
by its length (in centimeters). One megaohm centimeter equals 10 000 ohm meters.
megaparsec (Mpc)
the longest distance unit in common use, the megaparsec is used by astronomers
studying the most distant quasars and galaxies. One megaparsec equals one
million parsecs, 3.2616 million light
or 30.857 x 1018 kilometers (30.857 zettameters).
megapascal (MPa, MP)
a common metric unit of pressure or stress equal to one million pascals
or one newton per square millimeter. One megapascal
equals 10 bars or approximately 145.038 pounds
per square inch (lbf/in2 or psi) or 20 885.5 pounds (10.443 U.S.
tons) per square foot. The symbol MP is used fairly commonly in engineering,
but it is not correct: use MPa.
a unit used to describe the size or resolution of an image or of a digital
camera. One megapixel is one million pixels (picture elements, or “dots”).
For example, a rectangular image 1000 pixels by 1000 pixels is comprised of
one megapixel.
megapond (Mp)
a metric unit of force equal to 1000 kilograms
of force
(kgf). The megapond also equals 9806.65 newtons,
or 2204.6226 pounds of force in the traditional English system. Although it
is considered obsolete, the megapond is still used sometimes by engineers
in Europe, especially in Germany.
megaton (Mton or Mt)
a unit of energy used for measuring the energy of an explosion, especially
a nuclear explosion. Supposed to be the amount of energy released by the explosion
of one million (short) tons of TNT, the megaton is defined to equal 4.18 x
1015joules (4.18 petajoules), 1.16
billion kilowatt hours, or roughly 4 trillion Btu.
megatonne (Mt)
a metric unit of mass or weight equal to one million metric tons (tonnes),
one teragram (Tg), or about 2.2046 billion pounds.
megawatt (MW)
a common metric unit of power. One megawatt is equal to one million watts,
about 1341.02 horsepower, or 947.817 Btu
per second.
megawatt hour (MW·h)
a metric unit of energy, especially electrical energy. One megawatt hour
equals exactly 3.6 gigajoules (GJ), about 3.412
million Btu, or about 2.655 billion foot pounds.
megawatt day (MW·d or MWD)
a unit of energy used in the nuclear power and nuclear weapons industries.
One megawatt day equals exactly 24 megawatt hours, 86.4 gigajoules
(GJ), about 81.89 million Btu, or about 63.7
billion foot pounds.
megayear (Myr or Ma)
a unit of time equal to one million years.
a common unit of electric resistance equal to one million ohms.
This simplified spelling of megaohm is approved by the Institute of
Electrical and Electronics Engineers (IEEE).
A traditional distance unit in German speaking countries, the meile is much
longer than the mile units of western Europe. Typically the meile was equal
to 4000 klafters (fathoms) or 24 000 fuß
(German feet). In Austria this came to 7586 meters (4.714 miles); in northern
Germany it was 4.6805 miles or 7532.5 meters. A version of the meile called
the geographische meile was defined to equal exactly 4 (Admiralty)
nautical miles (24 320 feet,
4.6061 miles, or 7412.7 meters). The geographische meile was designed to equal
1/15 degree [2] or 4/3 league.
See also mil [4], the Scandinavian version of this unit.
a unit of perceived musical pitch, originally defined by Stevens, Volkmann,
and Newmann in 1937. Our perception of musical pitch is complex. Although
tones of higher frequency are perceived as being higher in pitch, tones separated
by equal intervals (frequency ratios), such as octaves, will not be perceived
as being equally spaced in pitch. A pure tone of frequency 1000 hertz, at
a sound level 40 decibels above the faintest sound a listener can hear, is
defined to have a pitch of 1000 mels, and tones perceived as being equally
spaced in pitch are separated by an equal number of mels. Because perceptions
of pitch depend on a number of factors other than frequency, it is not possible
to give a straightforward conversion between hertz and mels. For tones above
1000 hertz the perceived pitch in mels is lower than the frequency in hertz;
a 10-kilohertz tone is perceived at around 3000 mels. For tones lower than
1000 hertz the perceived pitch is a little higher than the frequency in hertz.
a huge bottle of champagne, holding about 18 liters.
Mercalli intensity scale
an empirical scale for rating the effects of an earthquake, as opposed to
its strength (see magnitude [2] above). Mercalli estimates are stated
as Roman numerals (I-XII) to avoid confusion with magnitude estimates on the
Richter scale. The scale is named for the Italian geologist Giuseppe Mercalli
(1850-1914), who devised the first version in 1902; the modified
used in the U.S. and Canada was developed by Charles F. Richter
in 1956.
mercantile pound (lb merc)
a historic English unit of weight, the mercantile pound (libra mercatoria)
was the commercial predecessor of the avoirdupois pound
[1]. Used from about 1100 to 1300, the mercantile pound contained
15 troy ounces [2] or 7200 grains.
This is equivalent to about 1.0286 avoirdupois pounds or 466.55 grams.
an ending meaning “-parted,” added to a number to create an adjective. Thus
“8-merous” means “having 8 parts.” The suffix, frequently used by botanists,
is derived from the Greek meros, “part.”
a traditional unit used to measure the fineness of woven products such as
fishing nets, fencing fabric, window screening, etc., equal to the number
of strands per inch. For n mesh fabric,
the distance between strands is 1/n inch or 25.4/n millimeter.
a unit of metabolism. Metabolism, the sum of all the processes going on
in the body to sustain life, is measured in units of power expended per unit
of body surface area. One met is the metabolism of a seated, resting person,
equal to about 58.15 watts per square meter
(W/m2) or 13.89 calories per second
per square meter (cal/m2·s) regardless of the person’s size.
Measurements of human metabolism generally fall in the range 0.8-3.0 met,
although athletes can achieve 10 met or more.
meter or metre (m)
the metric and SI base unit
of distance. Originally, the meter was designed to be one ten-millionth of
a quadrant, the distance between the Equator
and the North Pole. (The Earth is difficult to measure, and a small error
was made in correcting for the flattening caused by the Earth’s rotation.
As a result, the meter is too short by a bit less than 0.02%. That’s not bad
for a measurement made in the 1790’s.) For a long time, the meter was precisely
defined as the length of an actual object, a bar kept at the International
Bureau of Weights and Measures in Paris. In recent years, however, the SI
base units (with one exception) have been redefined in abstract terms so they
can be reproduced to any desired level of accuracy in a well-equipped laboratory.
The 17th General Conference on Weights and Measures in 1983 defined the meter
as that distance that makes the speed of light in a vacuum equal to exactly
299 792 458 meters per second. The speed of light in a vacuum, c,
is one of the fundamental constants of nature. Since c defines the
meter now, experiments made to measure the speed of light are now interpreted
as measurements of the meter instead. The meter is equal to approximately
1.093 613 3 yards, 3.280 840 feet,
or 39.370 079 inches. Its name comes from the
Latin metrum and the Greek metron, both meaning “measure.”
The unit is spelled meter in the U.S. and metre in Britain;
there are many other spellings in various languages (see Spelling
of Metric Units
another name for the atmo-meter.
meterlambert or meter-lambert
another name for the nit, an MKS
unit of luminance equal to one candela per
square meter. The name is coined by analogy with the footlambert.
meter per second (m/s)
the metric and SI unit of speed or velocity. One
meter per second is equal to exactly 3.6 kilometers per hour (km/h) or about
2.236 936 miles per hour or 3.280 840 feet per second.
a large wine bottle holding about 6 liters, 8 times the volume of a regular
Metonic cycle
a unit of time equal to 19 years, used in astronomy in predicting the phases
of the Moon. By coincidence, 19 years is equal to 6939.602 days and 235 lunar
months is equal to 6939.689 days, just 125 minutes longer. As a result, the
phases of the Moon repeat almost exactly after 19 years. (Since 19 years can
contain either 3 or 4 leap days, the recurrence isn’t always exact as to the
day of the month.) Many lunar calendars, such as the Chinese and Jewish calendars,
share this 19-year cycle. The cycle is named for the ancient Greek astronomer
Meton, who first used it for predictions around 433 BC.
metric carat
the current internationally-recognized carat,
equal to exactly 200 milligrams.
metric grain
a unit of mass sometimes used by jewelers, equal to 50 milligrams or 1/4
carat. This unit is often used for pearls and
is sometimes called the pearl grain.
metric horsepower
a unit of power, defined to be the power required to raise a mass of 75
kilograms at a velocity of 1 meter per second. This is approximately 735.499
watts or 0.986 32 horsepower.
The unit is known in French as cheval vapeur, in Spanish as caballo
de vapor, and in German as Pferdestärke.
metric hundredweight
an informal unit of mass equal to 50 kilograms or approximately 110.231
pounds, close to the traditional British hundredweight
of 112 pounds. This unit is also known by its German name, the zentner,
or (in English) the centner.
metric mile
an informal unit of distance used mostly in athletics. The metric mile is
equal to 1500 meters or 1.5 kilometers (approximately 0.932 057 statute mile
or 4921.26 feet). In U.S. high school competition
the term is sometimes used for a race of 1600 meters (0.994 194 miles or 5249.34
metric pound
an informal name for a mass of 500 grams (0.5 kilogram or 1.1023 pound).
metric quintal
a unit of mass equal to 1 decitonne, 100 kilograms or about 220.462 pounds.
See quintal for a more complete description.
metric slug
see TME or hyl.
metric ton (t or MT)
an alternate name for the tonne. In the United
States, the Department of Commerce recommends that the tonne be called the
metric ton to distinguish it clearly from the traditional American ton. The
proper symbol for the unit is simply t.
metric ton unit (mtu)
a unit of mass used in mining to measure the mass of the valuable metal
in an ore. Customarily, the metric ton unit is defined to be one metric ton
of ore containing 1% metal, but it is the metal, not the ore, that is being
measured. Thus the unit is really a unit of mass equal to 10 kilograms (22.0462
the symbol for one million electronvolts.
Thanks to Einstein’s equation E = mc2 equating mass wth energy,
the MeV can be regarded either as a unit of energy equal to 160.217 646 2
femtojoules, or as a unit of mass equal to
1.782 662 x 10-27 gram or 0.001 073 544 atomic
mass unit
. This is 1.956 951 times the mass of the electron (me).
MFD, mfd
common but incorrect symbols for the microfarad (see below). The correct
symbol is µF, or mcF if µ is not available.
an obsolete symbol for “milligram atom”, an equally obsolete name for the
millimole (mmol).
an abbreviation for millions of gallons per day (Mgal/d), a unit used in
reservoir management to express the rate at which water is withdrawn, or could
be withdrawn, for drinking or for some other purpose. 1 Mgd equals approximately
3.785 43 megaliters per day, or 3785.43 cubic meters per day, or 133 681 cubic
feet per day.
symbol for milligram per deciliter, a unit used in U.S. medicine to measure
the concentration of cholesterol and other substances in the blood. 1 mg/dl
equals 0.01 grams per liter (g/L). Internationally, the SI
unit for data of this type is millimoles per liter (mmol/L); see the table
of SI Units for Clinical Data for
conversions of many common measurements.
an obsolete symbol for “milligram equivalent”, an equally obsolete name
for the milliequivalent (mEq).
symbol for milligram per kilogram, a unit used in medicine to measure dosage
rates. 1 mg/kg is equivalent to 10-6 g/g or 1 part per million
based on the patient’s body weight.
symbol for the milligon (milligrad), a unit of angle measure equal to 0.001
gon, 10-5 right angle, 0.0009°,
3.24 seconds of arc, or 15.708 microradians (µrad). Surveying equipment
is often marked in “mgons.”
an older name for the siemens, which is
defined to be the reciprocal of the ohm. In case
you didn’t notice already, “mho” is “ohm” spelt backwards.
an informal name for the microgram, pronounced “mike.”
a unit used in computer science in programming mice and similar input devices.
One mickey is the length of the smallest detectable movement of the mouse.
This depends on the equipment. Typical values are in the range 1/200 to 1/300
inch or roughly 0.1 millimeter. Obviously, the
name comes from the Disney cartoon character Mickey Mouse.
micro- (µ- or mc- or u-)
a metric prefix meaning 10-6 (one millionth). The prefix comes
from the Greek prefix mikro-, meaning small. In print the prefix
is sometimes abbreviated mc- or u- when the Greek letter mu (µ) is not
microampere (µA)
a unit of electric current equal to 10-6ampere.
microarcsecond (µas)
a unit of angle measurement sometimes used in astronomy. The microarcsecond
equals 10-6 arcsecond or about 4.8481 picoradian.
microbar (µbar)
a CGS unit of pressure equal to 0.001 millibar,
0.1 pascal, 1 dyne
per square centimeter (1 barye), or about 0.002
089 pounds per square foot. The microbar is used commonly in acoustics and
sound engineering.
microcurie (µCi)
a common unit of radioactivity. The microcurie equals 10-6curie
or 37 kilobecquerels; this corresponds
to a radioactivity of 37 000 atomic disintegrations per second.
microdegree (µdeg) [1]
a unit of angle measure equal to a millionth of a degree or exactly 36 milliarcseconds.
microeinstein (µE)
a unit of light energy concentration used in measuring the flux or density
of light or any form of electromagnetic radiation. The microeinstein is equal
to 10-6einstein or one micromole
of photons. The density of photosynthetically active radiation, for example,
is reported in microeinsteins per second per square meter (µE/s·m2).
microequivalent (µEq or µeq)
a unit of relative amount of substance equal to 10-6equivalent
. This unit is used, for example, in stating the concentrations
of ions in drinking water.
microfarad (µF)
a common unit of electric capacitance equal to 10-6farad.
microflick (µf)
a unit of spectral radiance used in optical and communications engineering,
equal to 10-6flick, or 1 microwatt
per steradian per square centimeter of surface per micrometer of span in wavelength.
This is mathematically equivalent to 10 milliwatts per steradian per cubic
microgram (µg or mcg)
a metric unit of mass equal to 0.001 milligram (mg) or one millionth of
a gram. Ingredients of drugs and vitamins are often stated in micrograms.
microgray (µGy)
a unit of radiation dose equal to a millionth of a gray or 0.1 millirad.
Small doses of this size are often provided by natural sources in the environment.
microinch (µin)
a traditional unit of distance equal to 10-6 inch, 0.001 mil,
or 25.4 nanometers (nm). The microinch is used rather widely to state the
roughness of optical surfaces, precise tolerances in machining, and for other
industrial purposes.
microliter (µl, µL, mcl, or mcL)
a metric unit of volume equal to 0.001 milliliter or 1 cubic millimeter
(mm3). Microliters are used in chemistry and medicine to measure
very small quantities of liquid. This unit has also been called the lambda.
micrometer (µm)
a common metric unit of distance equal to 0.001 millimeter or about 0.039
370 mil. The name micron is also used for this unit.
micromicro- (µµ-)
an obsolete metric prefix denoting 10-12. The prefix has been
replaced by pico- (p-).
micromicrofarad (µµF or mmfd)
an older name for the picofarad (10-12farad).
Though it is obsolete now, this name is still seen marked on many capacitors.
micromicron (µµ)
a former name for a millionth of a micron, that is, 10-12 meter.
The name bicron was also used for this unit, which is now called the
picometer (pm).
micromole (µmol)
a unit of amount of substance equal to a millionth of a mole (see below).
This unit is used very commonly in biochemistry, since a mole of a large organic
molecule can be quite a large amount.
micron (µ) [1]
a metric unit of distance equal to one millionth of a meter. “Micron” is
simply a shorter name for the micrometer. In 1968 the CGPM
decided to drop the micron as an approved unit and recommend that micrometers
be used instead. Microns, however, are still in common use.
micron (µ) [2]
an informal unit of pressure widely used in vacuum technology. In this use,
a micron is a micron of mercury, that is, 0.001 mm Hg or approximately 1.333
microbars (µbar or µb) or 133.3 millipascals
(mPa). For all practical purposes, 1 micron is identical to 1 millitorr
micronewton (µN)
a unit of force equal to a millionth of a newton
or 0.1 dyne. The unit is often used in astronautical
engineering to describe the tiny forces applied to spacecraft to adjust their
attitudes in space.
micropascal (µPa)
an SI unit of pressure equal to 10-6pascal or
1 micronewton per square meter. This very
small unit is used to measure the pressure of sound waves.
micropoise (µP, µPo, or µPs)
a unit of dynamic viscosity used primarily for describing the viscosities
of gases. One micropoise equals 10-6 poise or 10-7pascal
second (Pa·s).
microrad (µrad)
a unit of radiation dose equal to a millionth of a rad
or 10 nanograys.
microradian (µrad)
a unit of angle measure equal to 10-6radian.
The microradian equals about 0.208 533 milliarcsecond (mas).
microrem (µrem)
a unit of effective radiation dose equal to a millionth of a rem
or 10 nanosieverts. Doses in this range are
much smaller than those provided by natural sources of radioactivity in the
microsecond (µs or µsec)
a unit of time equal to a millionth of a second.
microsievert (µSv)
a unit of radiation dose equal to 10-6sievert
or 0.1 millirem. The radiation doses resulting
from exposure to natural sources such as radon gas in the atmosphere are often
measured in this unit.
microstrain (µstrain)
a common engineering unit measuring strain. An object under strain is typically
deformed (extended or compressed), and the strain is measured by the amount
of this deformation relative to the same object in an undeformed state. One
microstrain is the strain producing a deformation of one part per million
microtesla (µT or mcT)
a common unit of magnetic field intensity equal to 10-6tesla.
The unit is widely used to measure the strength of electromagnetic fields
generated by powerlines or electronic equipment. By comparison, the strength
of the Earth’s own magnetic field at the surface is about 50 microteslas.
One microtesla equals 0.01 gauss.
microvolt (µV or mcV)
a unit of electric potential equal to 10-6volt.
This unit is used in cardiology and other medical fields to measure the small
potentials within the nervous system.
an informal unit of volume for beer used in many Australian pubs. A middy
is generally 285 milliliters (or 10 British fluid ounces), larger than a pony
but smaller than a schooner.
the traditional Italian mile. The miglio equals 1628 yards, which is 0.925
English mile or about 1488.6 meters. This is 32 yards (29.3 meters) shorter
than the classical Roman mile.
miil or mijl
alternate spellings for the Scandinavian mil [4] (see below).
mil [1]
a unit of distance equal to 0.001 inch: a “milli-inch,” in other words.
Mils are used, primarily in the U.S., to express small distances and tolerances
in engineering work. One mil is exactly 25.4 microns, just as one inch is
exactly 25.4 millimeters. This unit is also called the thou. With the
increasing use of metric units in the U.S., many machinists now avoid the
use of “mil” because that term is also a handy slang for the millimeter.

mil [2]
a unit of angle measure, used in the military for artillery settings. At
one time the U. S. Army used a mil equal to 1/1000 of a right angle, 0.1 grad,
0.09°, or 5.4 arcminutes (often written 5.4 moa; see “moa” below). Later
this was changed to 1/1600 right angle, or 0.05625° (3.375 moa). In target
shooting, the mil is often understood to mean 0.001 radian
or 1 milliradian, which is about 0.0573° or 3.43775 moa. In Britain,
the term angular mil generally refers to the milliradian. 1 milliradian
corresponds to a target size of 10 millimeters at a range of 10 meters, or
3.6 inches at 100 yards.
mil [3]
a common slang name for the milliliter (mL) or the millimeter (mm).
mil [4]
in Scandinavia, the mil, pronounced like “meal” in English, is a traditional
distance unit considerably longer than Roman or English miles. In Denmark,
the traditional mil was 24 000 Danish feet, which is 4.6805 miles or 7.5325
kilometers (this is the same as the north German meile; see above). The Danish
mil has sometimes been interpreted as exactly 7.5 kilometers (4.6603 miles).
In Sweden, the traditional mil was 36 000 Swedish feet, which is 6.641 miles
or 10.687 kilometers. In Sweden and Norway the mil is now interpreted as a
metric unit equal to exactly 10 kilometers (6.2137 miles). See also sjømil.
mil [5]
an alternate spelling of the mill [1] (see below).
mile (mi) [1]
a traditional unit of distance. The word comes from the Latin word for 1000,
mille, because originally a mile was the distance a Roman legion
could march in 1000 paces (or 2000 steps, a
pace being the distance between successive falls of the same foot). There
is some uncertainty about the length of the Roman mile. Based on the Roman
foot of 29.6 centimeters and assuming a standard pace of 5 Roman feet, the
Roman mile would have been 1480 meters (4856 feet); however, the measured
distance between surviving milestones of Roman roads is often closer to 1520
meters or 5000 feet. In any case, miles of similar lengths were used throughout
Western Europe. In medieval England, several mile units were used, including
a mile of 5000 feet (1524 meters), the modern mile defined as 8 furlongs (1609
meters), and a longer mile similar to the French mille (1949 meters). None
of these units corresponded with the Scottish mile (1814 meters) or the Irish
mile (2048 meters). In 1592, Parliament settled the question in England by
defining the statute mile to be 8 furlongs,
80 chains, 320 rods
[1], 1760 yards or 5280 feet.
Using the international definition of the foot as exactly 30.48 centimeters,
the international statute mile is exactly 1609.344 meters. (In technical U.S.
usage, the statute mile is defined in terms of the survey foot and equals
about 1609.3472 meters; this unit is called the survey mile). In athletics,
races of 1500 or 1600 meters are often called metric miles.
See also nautical mile.
mile (mi) [2]
an informal name for mile per hour, sometimes seen on U.S. road signs with
markings such as “Speed Limit 25 miles.”
mile per gallon (mi/gal or mpg) [1]
the unit customarily used in the United States to measure the fuel efficiency
of motor vehicles. 1 mile per U.S. gallon
[1] equals about 0.4252 kilometers per liter. In most other countries, however,
the usual measure of fuel consumption is liters per 100 kilometers; x
miles per U.S. gallon is equal to 235.215/x liters per 100 kilometers.

mile per gallon (mi/gal or mpg) [2]
the unit formerly used in Britain, Canada, Australia, and other British
Commonwealth nations to measure the fuel efficiency of motor vehicles, analogous
to the U.S. unit but based on the Imperial gallon
[3]. Although still used sometimes, this unit has been replaced officially
by liters per 100 kilometers; x miles per Imperial gallon is equal
to 282.481/x liters per 100 kilometers. One mile per Imperial gallon
is equal to about 0.8327 mile per U.S. gallon.
mile per hour (mi/h or mph)
a traditional unit of velocity. One mile per hour equals exactly 22/15 feet
per second, approximately 1.609 kilometers per hour (km/h), or exactly 0.447
04 meter per second (m/s).
a mil-foot is a section of wire one foot long and one mil in diameter; this
would be a unit of volume equal to about 0.0377 cubic inches or 0.6178 cubic
centimeters. However, the unit is used primarily in statements of resistivity
in ohms per mil-foot or of density in pounds
per mil-foot. The unit is also called the circular mil-foot.
the traditional Portuguese mile, one of the “longest miles” of western Europe
at 2282.75 yards (1.297 statute miles or 2087.3 meters).
military pace
another name for a step. In the U.S. Army,
the military pace is defined to be exactly 30 inches (76.2 centimeters) for
ordinary “quick time” marching and 36 inches (91.44 centimeters) for double
time marching. The same definitions are generally used by marching bands.
mill [1]
an informal unit of quantity or of proportion, equal to 0.001. When Congress
established the U. S. monetary system in 1791, it provided for 10 mills to
the cent and 100 cents to the dollar; thus the mill was an amount of money
equal to $0.001. Although the mill is unfamiliar now as a monetary unit, it
has come to represent a one thousandth part as a proportion. Many towns in
the United States set their property tax rates in mills, for example.
mill [2]
slang for one million.
the traditional Spanish mile, equal to 5000 pies (Spanish feet) or 8 estadios.
This is about 1392 meters, 4567 feet, or 0.865
statute mile.
mille [1]
the traditional French mile, equal to 1000 toises.
This is equal to about 6394.4 feet, 1.211 statute
mile, or 1949 meters. In modern France, the mille sometimes means the
nautical mile (mille marin),
equal to exactly 1852 meters.
mille [2]
in French-speaking Canada, the English statute mile of 5280 feet
(1609.344 meters).
mille [3]
the Latin word for 1000, sometimes used in English in very learned or literary
a unit of quantity equal to 1000.
a traditional unit of time equal to 1000 years. The plural is millennia
or sometimes millenniums.
milli- (m-)
a metric prefix meaning 0.001 (one thousandth). The prefix was coined from
the Latin number mille, one thousand.
milliampere (mA)
a common unit of electric current equal to 0.001 ampere.
milliampere hour (mA·h)
a common unit of electric charge, used (for example) in stating the capacity
of batteries for cell phones and other electronic equipment. One milliampere
hour is the charge accumulated by a current of 1 milliampere in 1 hour; this
is equal to exactly 3.6 coulombs (C).
milliarcsecond (mas)
a unit of angular measure commonly used in astronomy. One milliarcsecond
is equal to 0.001 arcsecond, 0.277 77 microdegrees, or 4.848 137 nanoradians.
milliard [1]
a unit of quantity equal to 109, which is what Americans call
a billion. See Using Numbers and Units for more
on the “billion” controversy.
milliard [2]
a unit of volume used by engineers to describe a large quantity of water.
One milliard equals one cubic kilometer, which is 1 billion (109)
cubic meters or about 810 767 acre feet.
millibar (mb)
a common metric unit of atmospheric pressure, equal to 0.001 bar, 100 pascals,
1000 dynes/cm2, about 0.0295 inches
(0.7501 millimeters) of mercury, or about 0.014 504 lb/in2. A millibar
is the same thing as a hectopascal (hPa), and some weather
agencies have replaced the millibar with the hectopascal in an effort to conform
with the SI. However, many meteorologists resist this
change and continue to use millibars. In fact, an appropriate SI unit for
atmospheric pressure would be the kilopascal (10 millibars or 0.145 038 lb/in2).
millicandela (mcd)
a unit of light intensity equal to 0.001 candela.
The intensity of the light-emitting diodes (LEDs) used in electronics are
stated in millicandelas.
millicurie (mCi)
a common unit of radioactivity. One millicurie represents radioactivity
at the rate of 37 million atomic disintegrations per second, that is, 37 megabequerels.
millidegree (mdeg) [1]
a unit of angle measure equal to 0.001° or exactly 36 arcseconds.
millidegree (mdeg) [2]
a unit of temperature equal to 0.001°, usually meaning 0.001 °C.
milliequivalent (mEq or meq)
a unit of relative amount of substance commonly used in chemistry. One mEq
equals 0.001 equivalent weight.
a former name for the tonne or metric ton.
This name, obsolete now, was used in Britain to avoid confusion with the British
long ton.
millifarad (mF)
a common unit of electric capacitance equal to 0.001 farad.
milligal (mGal or mgal)
a unit of acceleration used in geology to measure subtle changes in gravitational
acceleration. One milligal equals 10 micrometers per second per second, or
10-5 meters per second per second. The unit should really be called
the milligalileo.
milligauss (mG)
a unit of magnetic flux density equal to 0.001 gauss,
0.1 microtesla, or 100 nanoteslas. The magnetic
fields generated by power lines and electronic equipment are often measured
in milligauss.
milligram (mg)
a very common metric unit of mass equal to 0.001 gram
or 1000 micrograms (µg or mcg). One milligram equals approximately 0.015432
grain or 35.274 x 10-6 ounce.
milligram per deciliter (mg/dl or mg/dL)
a conventional unit in medicine for measuring concentrations of cholesterol
and many other substances in the blood. Internationally, the SI
unit for data of this type is millimoles per liter (mmol/L); see the table
of SI Units for Clinical Data for
conversions of many common measurements.
milligray (mGy)
a common unit of radiation dose equal to 0.001 gray,
0.1 rad, or 1 millijoule of energy per kilogram
of matter. Because the gray itself is such a large unit, many practical radiation
measurements are made in milligrays. In particular, the exposures cause by
X-ray equipment are typically in the milligray range.
millihenry (mH)
a common metric unit of electric inductance equal to 0.001 henry.
an informal name (pronounced “millig”) for the millimeter of mercury (see
millihorsepower (mhp)
a unit of power equal to 0.55 foot-pound per second or 0.7457 watt.
This unit is commonly used to state the power of small electric motors.
millijoule (mJ)
a common metric unit of work or energy equal to 0.001 joule
or 104ergs.
a unit used in nuclear engineering to describe the “reactivity” of a nuclear
reactor. One milli-k is a reactivity of 0.001 or 0.1%; the origin of the name
is that k is a common symbol for reactivity. This unit was introduced
in the Canadian nuclear power industry. For a discussion of reactivity, see
millikelvin (mK)
a unit of temperature equal to 0.001 kelvin
or 0.001 degree Celsius (°C). This unit
is used mostly by scientists investigating substances cooled very close to
absolute zero.
millilambert (mLb)
a common metric unit of illumination equal to 0.001 lambert
or 10 lux (lx).
millilux (mlx)
a metric unit of illumination equal to 0.001 lux.
The natural illumination at night is measured in millilux.
milliliter (ml or mL)
a very common metric unit of volume. One milliliter equals 0.001 liter,
exactly one cubic centimeter (cm3 or cc), or approximately 0.061
023 7 cubic inch or 16.231 U.S. minims (see below). The milliliter is used
almost entirely for measuring the volumes of liquids, with solids being measured
in cubic centimeters. Note: until 1964 the milliliter was equal to 1.000 028
cubic centimeters; see liter for a discussion
of this history.
millimass unit (mu or mmu)
a unit of mass equal to 0.001 atomic mass unit,
used in physics and chemistry. This unit is also called the millidalton
(mDa). The millimass unit is an SI unit, but its proper
SI symbol is mu, not the older symbol mmu.
millimeter (mm)
a very common metric unit of distance. One millimeter equals 0.001 meter,
0.1 centimeter, about 0.039 370 inch, or 39.370 mils.
millimeter of mercury (mm Hg)
a unit of pressure equal to the pressure exerted at the Earth’s surface
by a column of mercury 1 millimeter high. When a traditional mercury barometer
is used, the pressure is read directly as the height of the mercury column
in millimeters. One millimeter of pressure is equivalent to approximately
0.03937 in Hg, 0.01933 lbf/in2, 1.333
millibars, or 133.3 pascals.
In medicine, blood pressure is traditionally recorded in mm Hg. In engineering,
the millimeter of mercury is often replaced by the torr,
the two units being equal to within 1 part per million. Hg, the chemical symbol
for mercury, is taken from the Latin hydrargyrum, “water-silver,” describing
the silvery liquid metal.
millimeter of water (mmH2O, mm WC, mm
CE, mm WS)

a unit of pressure equal to the pressure exerted at the Earth’s surface
by a column of water 1 millimeter high. This is a small pressure, about 9.8067
pascals, 0.098 067 millibars,
0.03937 inch of water, or 0.204 pounds per square
foot. The French symbol is mm CE (colonne d’eau), and the German symbol
is mm WS (Wassersäule).
millimeter of water gauge (mm WG)
another common name for the millimeter of water column. The word “gauge”
(or “gage”) after a pressure reading indicates that the pressure stated is
actually the difference between the absolute, or total, pressure and the air
pressure at the time of the reading.
millimicro- (mµ-)
an obsolete metric prefix denoting 10-9 or one billionth. This
prefix has been replaced by nano- (n-).
millimicron (mµ)
a former metric unit of distance equal to 0.001 micron or 10-9
meter. The millimicron has been replaced by its equivalent, the nanometer
millimole (mmol)
a very common unit of amount of substance equal to 0.001 mole (see below).
millimole per liter (mmol/l or mmol/L)
the SI unit in medicine for measuring concentrations
of cholesterol and many other substances in the blood. A table
is provided for conversion of conventional units, such as milligrams per deciliter
(mg/dL), to SI units.
a traditional unit of advertising. One milline equals the height of a line
of “agate” type (5.5 points, or about 2 mm) times the width of a column times
one million copies of the publication.
millinewton (mN)
a metric unit of force equal to 0.001 newton,
100 dynes, or about 0.101 972 gram of force
a unit used in British nuclear engineering to describe the “reactivity”
of a nuclear reactor. One millinile is a reactivity of 10-5. For
a discussion of reactivity, see inhour.
millioctave (mO)
a unit used in music to describe the ratio in frequency between notes. The
difference between two frequencies in millioctaves is equal to 1000 times
the base-2 logarithm of the ratio between the two frequences. One millioctave
equals exactly 1.2 cents [3] or about 0.30103
savart. If two notes differ by 1 millioctave,
the ratio between their frequencies is 21/1000 or approximately
1.000 6934.
millioersted (mOe)
a name sometimes used for the milligauss as a unit of magnetic flux density.
milliosmole (mOsm)
a unit of osmotic pressure, equal to 0.001 osmole,
commonly used in biology and medicine.
milliparsec (mpc)
a unit of distance in astronomy equal to 0.001 parsec.
Used in studying crowded parts of the universe such as globular clusters and
galactic centers, the milliparsec is equal to about 206.265 astronomical
, 11.913 light days, or 30.8568 terameters (30.8568 x 109
kilometers or 49.6486 million miles).
millipascal (mPa)
an SI unit of pressure equal to 0.001 pascal
or 1 millinewton per square meter. This very
small unit is used to measure the pressure of sound waves.
millipascal second (mPa·s)
an SI unit of dynamic viscosity equal to the centipoise
(cP). This unit is gradually replacing the centipoise in many contexts.
milliphot (mph)
a unit of illuminance or illumination equal to 0.001 phot
or 10 lux.
millipoise (mP, mPs, or mPo)
a metric unit of dynamic viscosity equal to 0.001 poise
or 0.1 millipascal second (mPa·s).
millirad (mrad)
a unit of radiation dose equal to 0.001 rad
or 10 micrograys.
milliradian (mrad)
a unit of angle measure equal to 0.001 radian. The milliradian equals about
0.057 296°, 3.437 75 arcminutes, or 3″ 26.265′. In Britain this unit
is often called the angular mil.
millirem (mrem)
a common unit of radiation dose equal to 0.001 rem
or 10 microsieverts (µSv). A millirem
is roughly the radiation dose you would receive from wearing a luminous dial
watch for a year.
millisecond (ms or msec)
a common unit of time equal to 0.001 second.
millisiemens (mS)
a common unit of conductance equal to 0.001 siemens
or 1 milliampere of current per volt
of potential difference. The millisiemens is often used to measure the salinity
of seawater or brackish water, since adding salt to water makes it much more
conductive of electricity.
millisievert (mSv)
a unit commonly used to measure radiation dose. One millisievert equals
0.001 sievert or 0.1 rem.
millitesla (mT)
a common unit of magnetic field intensity equal to 0.001 tesla
or 10 gauss. Since the tesla is quite a large
unit, many practical measurements are made in milliteslas.
millivolt (mV)
a common unit of electric potential equal to 0.001 volt.
milliwatt (mW)
a common unit of power equal to 0.001 watt.
milliwatt hour (mW·h)
a common metric unit of work or energy, representing the energy delivered
at a rate of one milliwatt for a period of one hour. This is equivalent to
exactly 3.6 joules (J) of energy, or about
0.003 412 Btu, 0.859 846 (small) calories,
or about 2.655 foot pounds.
a historic unit of weight, originating in Babylonia and used throughout
the eastern Mediterranean. The mina is roughly comparable to the pound,
but over the centuries it varied quite a bit. In Babylonian times it was a
large unit, roughly 2 pounds, almost as much as a kilogram. The Hebrew mina,
frequently mentioned in the Bible, is estimated at 499 grams (1.10 pounds).
The Greek mina was equal to 100 drachmai
or 431 grams (0.95 pound). In Biblical times the mina was equal to 60 shekels,
and there were 60 minas in a talent.
miner’s inch
a traditional unit of water flow in the western United States. The unit
originally represented streamflow through an opening one inch
(25.4 mm) square at a specified distance below the surface of the water; this
distance varied from 4 to 6 inches. In Idaho, Kansas, Nebraska, New Mexico,
North and South Dakota, Utah, and Washington the miner’s inch is legally defined
to equal 9 gallons per minute or 1.2 cubic
feet per minute (about 34.07 liters per minute). In Arizona, California, Montana,
Nevada, and Oregon the definition is 1.5 cubic feet per minute (42.48 L/min).
In Colorado, the legal equivalent is 1.5625 cubic feet per minute (44.25 L/min).
See also water inch.
a suffix used to create small numbers. The number n-minex is 10-n,
which is 0.000…0001 with a total of n-1 zeros between the decimal
marker and the 1. Thus one millionth (0.000001), for example, is 6-minex.
See also dex and -plex.
minim (m or min) [1]
a traditional unit of volume used for very small quantities of liquids.
In pharmacy, the term drop traditionally meant
the same thing as 1 minim. The minim is defined to be 1/60 fluid
or 1/480 fluid ounce. The U. S. minim is equal to about 0.003 760
cubic inch or 61.610 microliters, while the British minim is equal to about
0.003 612 cubic inch or 59.194 microliters. As you might guess, the word comes
from the Latin minimus, small.
minim [2]
a unit of relative time in music equal to 1/2 whole note (a half note) or
1/4 breve.
an informal unit of volume for beer and other alcoholic beverages, used
mostly in Britain. A minipin is 1/2 of a polypin;
this is about 17 Imperial pints or 10 liters (roughly 2.64 U.S. gallons).
minute (′) [0]
a historic unit of proportion equal to 1/60. The Romans lacked our flexible
terminology for fractions; they followed Babylonian and Greek practice in
visualizing quantities as being divided into 60 parts, so they could express
fractions consistently in 60ths. A 60th part was called a pars minuta
(“first small part”) of the whole. For smaller fractions, a 60th
part was divided into 60 smaller parts, each called a pars minuta
(“second small part”). The minuta prima has come down
to us as “minute,” the minuta secunda is our “second,” and the prima
leads to the traditional symbol ′ being called a “prime.”
minute (min or ′ or m) [1]
a unit of time equal to 60 seconds or to 1/60 hour. The SI
specifies min as the symbol for the time unit and ′ as the symbol for
the minute of arc (see below).
minute (′ or m or moa) [2] or minute of arc or minute of
a unit of angular measure equal to 60 arcseconds or to 1/60 degree. This
unit is often called the arcminute to distinguish it from
the minute of time. There are 21 600 arcminutes in a circle. The SI
defines min as the symbol for the time unit (see above) and
recommends ′ as the symbol for the minute of arc. The symbol
moa is often used in target shooting. The international standard ISO
31 requires that angles be stated in degrees and decimal fractions of the
degree, without use of arcminutes and arcseconds.
minute (′ or m) [3]
a unit of angular measure used in astronomy. Astronomers measure right ascension
(see hour [2]) in time units by dividing the
equator into 24 hours instead of 360 degrees. This makes 1 minute of right
ascension equal to 15 arcminutes.
minute [4]
a unit of time equal to 1/60 day or 24 minutes in the modern sense. This
was the original definition of the minute as a unit of time. The modern definition
of 1/60 hour did not appear until the invention of mechanical clocks made
it practical to measure such small intervals of time.
minute (min or m or ′) [5]
a unit of sidereal time in astronomy; see sidereal
in medieval times, a unit of time equal to 1/10 hour, or 6 minutes in modern
terminology. This unit was divided into 4 moments (see below).
a traditional Hebrew unit of quantity equal to 10, the number of males aged
13 or over required for a Jewish worship service. (In many modern congregations,
both males and females can be included in a minyan.)
a unit of computing power equal to one million instructions per second.
An “instruction” is a single program command to the computer’s central processor.
In a particular computer, there is a definite relationship between the rate
at which instructions are processed, in mips, and the “clock speed” of the
processor, measured in megahertz (MHz). However,
this relationship varies considerably between computers, so it is usually
not meaningful to compare the mips rates of dissimilar machines. See also
megaflops (above).
a name used in colorimetry for the reciprocal
(MK-1). The word is an acronym for “micro-reciprocal
degree”; it is pronounced my-red, in two syllables.
symbol for one million international units.
Dosages of certain drugs, such as various forms of interferon, are commonly
stated in this unit.
see modified Julian day (below).
symbol for “meters from the Kelly bushing,” used in the oil and gas industry
to indicate the length of a bored well as measured from the large bushing
at the top of the shaft. Since the drilling is usually not exactly vertical,
this measurement will be larger than the actual depth of the bottom of the
well. The symbol mTVD is often used for true vertical depth.
a traditional unit of distance in East Africa, standardized under British
rule as 1/2 yard (18 inches,
or 45.72 centimeters). This unit is an African version of the cubit.
a common symbol for the meter kilopond,
a metric unit of torque equal to 9.806 65 newton
(N·m) or 7.233 01 pound feet.
Mlb, Mlbs (1)
common symbols for one million pounds. Although
“Mlbs” is seen frequently, symbols need not take the terminal -s
in the plural and this dictionary takes the position that they should not;
“Mlb” is correct. (Also note that the “l” is not capitalized
in the symbol for the pound.)
Mlb (2)
a traditional unit of mass for steam, equal to 1000 (not one million) pounds.
This is one of many uses of the Roman numeral M to represent a multiple of
1000; all these uses should be replaced by the metric prefix k- (kilo-).
a common symbol in chemistry for millimolar, that is, millimoles per liter.
The SI does not allow the use of this symbol.
an abbreviation for one million, seen in a few traditional units such as
those listed below. The abbreviation is meant to indicate one thousand thousand,
M being the Roman numeral 1000. However, MM actually
means 2000, not one million, in Roman numeration. Since the single letter
M is now used commonly for one million, the use of MM is confusing and strongly
an abbreviation for “by mass,” used in chemistry and pharmacology to describe
the concentration of a substance in a mixture or solution. 2% m/m means that
the mass of the substance is 2% of the total mass of the solution or mixture.
MMb, MMbo
symbols for one million barrels of oil; see megabarrel above.
symbols sometimes used in U.S. forestry for one million board
. One MMBF represents a volume of 83 333 cubic feet or 2360 cubic
meters. “BM” stands for “board measure.”
a traditional symbol for one million Btu (about
1.055 057 gigajoules (GJ)), a unit used widely
in the energy industry. This unit is also called the dekatherm.
a symbol for one million cubic feet (28 316.85 m3, or 28.316
85 megaliters). Similarly, MMMcf is used for one billion
cubic feet.
a symbol used in the natural gas industry for one million cubic feet of
gas equivalent (cfe). This is really an energy
unit equal to about 1.091 terajoules (TJ).
Similarly, MMMcfe is used for one billion cubic feet of gas
equivalent: 1.091 petajoules (PJ).
a symbol used for one million centipoises, a unit of dynamic viscosity. This is a jarring addition of an obsolete English prefix to a metric unit, and its use cannot be recommended. 1 MMcps equals 10 000 poises or 10 kilopoises (kP or kps).
an obsolete symbol for the microgram, mmg stands for “millimilligram,”
that is, 0.001 milligram. This symbol should never be used. The SI symbol
for the microgram is µg, and an acceptable alternate (often used in medicine)
is mcg.
symbol for the millimeter of mercury (see above), a unit of pressure equal
to 133.3 pascals.
an abbreviation for one billion (109), seen in a few traditional
units such as those mentioned above. The abbreviation is meant to indicate
one thousand thousand thousand, M being the Roman numeral
1000. However, MMM actually means 3000, not one billion, in Roman numeration.
symbol for one million standard cubic feet per day, the customary unit for
measuring the production and flow of natural gas. “Standard” means that the
measurement is adjusted to standard temperature (60 °F or 15.6 °C)
and pressure (1 atmosphere).
a “unit” traditionally used in printing to describe the page size of a book
or other publication. In traditional printing, large sheets are printed, folded,
and then cut to manufacture the book. After the cut is made, the sheet has
been divided into a certain number of “leaves.” Each leaf, folded at the spine
of the book, comprises two pages front and back. When sheets were cut to form
4, 8, or 12 leaves, the resulting pages were described as quarto (4to), octavo
(8vo) or duodecimo (12mo), respectively. Later, the suffix -mo from duodecimo
was made into a suffix that can be attached to any number to indicate the
number of leaves per sheet; thus 16mo indicates 16 leaves per sheet. Link:
book sizes,
from Bookbinding and
the Conservation of Books
, by Matt T. Roberts and Don Etherington, posted
by Stanford University.
an acronym for “minute of angle,” that is, for the arcminute (see minute
[2], above). This unit is commonly used in target shooting to express
the angular size of targets or the spacing between marks on a reticle (the
grid of lines seen in the eyepiece of a rifle). By coincidence, 1 moa is very
nearly equal to a target size of 1 inch at 100
yards; in fact, 1 moa = 1.047 20 inches at 100 yards or 10.4720 inches at
1000 yards. In metric units, 1 moa = 2.9089 centimeters at 100 meters.
modified Julian day (MJD)
a count of days used by astronomers, space agencies, and others. Astronomers
have long used the Julian day, a count
of days beginning at noon Universal Time January 1, 4713 BC, as a means of
specifying a date independent of all calendars. One problem with this is that
the numbers are large, more than 2.4 million, for current dates. Also, the
old astronomical custom of beginning a day at noon is awkward for converting
Julian dates to the ordinary calendar. To ease these problems, space engineers
introduced the modified Julian date, equal to the Julian date minus 2 400
000.5. The result is a count of days beginning at 0 hours (midnight) Universal
Time on 17 November 1858. Thus (for example) 0 hours 1 January 2005 is MJD
a unit of volume for raw cotton in the U.S. When cotton is harvested, machinery
is used to compact it into bundles called modules for transportation to the
gin. A cotton module is 8 ft by 8 ft by 20 ft, or 1280 cubic feet (about 36.25
cubic meters). This unit is essentially the same as the TEU,
the volume of a standard 20 ft container.
Mohs hardness scale
a 1-10 scale for estimating the hardness of a mineral, introduced by the
German geologist Friedrich Mohs (1773-1839) in 1812. To apply the scale, one
attempts to scratch the mineral with standard minerals assigned hardness numbers
as follows: diamond 10, corundum 9, topaz 8, quartz 7, orthoclase 6, apatite
5, fluorite 4, calcite 3, gypsum 2, and talc 1. If, for example, the mineral
is scratched by quartz but not by orthoclase, then its hardness is between
6 and 7.
another name for a half, from the French moitié.
Hebrew name for the lunar (synodic) month (see month [1] below).
This unit, 29.530 59 days, is crucial in the regulation of the Jewish lunisolar
molal (m), molar (M)
these notations, traditionally used by chemists to describe the concentration
of chemical solutions, often appear to be units of measurement. It’s easy
to get them confused. The term “molal” describes the concentration of a solution
in moles per kilogram of solvent (mol/kg), while “molar” describes a concentration
in moles per liter (mol/L). A solution described as 1.0 µM has a concentration
of 1.0 µmol/L. These units are not approved by the General Conference
on Weights and Measures. Their use is declining, but still substantial.
molar volume
a unit used by chemists and physicists to measure the volumes of gases.
The behavior of gases under ordinary conditions (not at very high pressures
or very low temperatures) is governed by the Ideal Gas Law. This law says
that the volume V of a gas is related to its temperature T and pressure P
by the formula PV = nRT, where n is the number of moles
of gas present and the gas constant R equals 8.314 joules
per mole per kelvin. The molar volume is the
volume one mole of gas occupies at standard temperature (273.16 kelvins, or
0 °C) and standard pressure (1 atmosphere,
or 101.325 kilopascals). The molar volume
is equal to 22.414 liters or 0.7915 cubic foot. (Occasionally the term “molar
volume” is used for the volume occupied by a mole of a substance which is
not a gas; in such cases the molar volume will be different for each substance.)
mole (mol)
the SI base unit of
the amount of a substance (as distinct from its mass or weight).
Moles measure the actual number of atoms or molecules in an object. An earlier
name is gram molecular weight, because one mole of a chemical
compound is the same number of grams as the molecular weight of a molecule
of that compound measured in atomic mass units.
The official definition, adopted as part of the SI system in 1971, is that
one mole of a substance contains just as many elementary entities (atoms,
molecules, ions, or other kinds of particles) as there are atoms in 12 grams
of carbon-12 (carbon-12 is the most common atomic form of carbon, consisting
of atoms having 6 protons and 6 neutrons). The actual number of “elementary
entities” in a mole is called Avogadro’s number after the
Italian chemist and physicist Amedeo Avogadro (1776-1856). Careful measurement
determines Avogadro’s number to be approximately 602.214 179 x 1021.
In the American system of naming big numbers, that’s 602 sextillion 214 quintillion
179 quadrillion, give or take about 50 quadrillion.
a medieval unit of time equal to 1/40 hour or 1.5 minutes. This meaning
has come down to us only as “a brief interval of time.” The moment was divided
into 12 ounces [4] of 7.5 seconds each.
momme [1]
a traditional Japanese weight unit corresponding to the Chinese mace (see
above). Jewelers continue to use the momme to measure the weight of cultured
pearls; for this purpose it equals exactly 3.75 grams (about 0.132 ounce)
or 18.75 carats. The unit is commonly pronounced
“mommy” in English.
momme [2]
a traditional unit used to measure the “weight” (density per unit area)
of silk. The measure is the weight in momme [1] of a standard strip
of silk 25 yards long by 1.49 inches
wide, an area of 1341 square inches or about 0.8652 square meter. This makes
the silk momme equal to about 3.62 grams per square yard or 4.33 grams per
square meter.
abbreviation for motor octane number. See octane
mondo point
another name for a millimeter, when used to measure shoe and boot sizes.
Ski boots, for example, are sized in mondo points.
month (mo or mon) [1]
a unit of time marked by the revolution of the Moon around the Earth. In
many traditional societies the appearance of the first tiny crescent moon
after the New Moon signaled the start of the month. This start of the month,
based on the first appearance of the Moon, is still proclaimed from mosques
in Islamic countries. Thus the lunar month is defined as
the average interval between two successive moments of New Moon. Astronomers
call this period the synodic month. Its length is 29.530
59 days.
month (mo or mon) [2]
a civil unit of time equal to approximately 1/12 year, but varying from
28 to 31 days [3]. The Sun and the Moon are our traditional time keepers,
but they are badly out of step with each other. A solar year equals approximately
12.368 lunar months. The large fraction in this number makes it difficult
to design a calendar with a whole number of months in each year. There are
at least three solutions to the problem:
[i] Use leap months. In the traditional Chinese
and Jewish calendars most years have 12 months, but some have a 13th month.
In these luni-solar calendars the length of the year varies
from 354 to 384 days.
[ii] Define 12 synodic lunar months to be a year and don’t worry
about the length of the year. This is the solution of the Islamic calendar.
Since the Islamic year has only 354 or 355 days, its length does not match
the cycle of the seasons.
[iii] Observe the solar year and let the months be 12 arbitrary
periods; don’t worry about the Moon. This is the solution adopted by Julius
Caesar, who established the civil calendar we use today. In this calendar,
all months have 30 or 31 days except the second month, February. February
has 28 days in ordinary years and 29 in leap years. See also year

a proposed unit in synthetic music, equal to one volt per octave. The unit
would honor Robert Moog (1934-2005), the inventor of the Moog synthesizer.
morgan (M)
a unit of genetic separation used in genetics and biotechnology. If two
locations on a chromosome have probability p of being separated during
recombination in a single generation, then the distance between those locations
is p morgan. In practice, measurements are made in centimorgans,
each centimorgan representing a 1% probability. The unit honors the American
geneticist Thomas Hunt Morgan (1866-1945), who received the Nobel Prize for
Medicine in 1933 for his pioneering work in studying the genetics of the fruit
fly Drosophila.
a traditional unit of land area in Northern Europe. “Morgen” means “morning,”
and most likely the unit arose as the area a yoke of oxen could plow in one
morning. The Dutch morgen, also used in Dutch colonies including old New York,
equals about 2.10 acres or 0.850 hectare.
In South Africa, this unit was defined to equal 10 246 square yards,
which is 2.1169 acres or 0.8567 hectare. In Scandinavia and northern Germany,
the morgen is a smaller unit equal to about 0.63 acre or 0.25 hectare (2500
square meters). The Prussian morgen, standardized at 2553.22 square meters,
was in common use during the nineteenth century. In Austria and southern Germany,
the morgen was often the same as a joch, typically
defined to be 0.5755 hectare or about 1.422 acres.
see mu (below).
mouse unit (MU or U)
an unofficial unit of toxicity used in pharmacology. A mouse unit is the
dose of a toxin that kills 50% of mice (that is, it is the LD50
dose for mice). Typically the mice are assumed to have a mass of 20 grams,
the toxin is administered by intraperitoneal injection, and mortality is measured
over a standard period that may vary according to the toxin. The size of the
mouse unit (in milliliters or international units) depends on the specific
symbols for the megapascal, a unit of pressure or stress (see above). MPa
is the correct symbol; MP should not be used.
abbreviation for million particles per cubic foot, a unit used to measure
concentration of dust particles, mostly in industrial settings. 1 mppcf is
equivalent to about 35.315 million particles per cubic meter. The symbol mp/f3
is sometimes used for this unit.
Italian abbreviation for the square meter (metro quadrato). Similarly,
cmq is a square centimeter and kmq is a square kilometer. These
are non-standard symbols; the correct symbol for the square meter is m2.
msl or MSL
abbreviation for “above mean sea level,” often seen in measurements
of altitude, as in m (msl) for meters above mean sea level
or ft msl for feet above mean sea level. “Mean sea level”
is defined to be the average height of the sea, for all stages of the tide,
as measured over a 19-year Metonic cycle (see above).
Spanish symbol for meters above sea level (metros sobre el nivel del mar), a unit of altitude.
a symbol commonly used in the oil industry for 1000 stock
tank barrels
symbol for “meters of seawater,” a conventional unit of pressure. The pressure
exerted by seawater varies slightly with temperature and salinity, but for
practical purposes the convention is that each meter imposes a pressure of
0.1 bar or 10 kilopascals
(about 0.102 kilograms of force per square centimeter or 1.45 pounds per square
inch). Sometimes the convention is that each meter is equivalent to 0.1 atmosphere
(0.1013 bar), which is practically the same thing. In English units, 1 msw
= 3.28 feet of seawater (fsw). Underwater pressure gauges are frequently calibrated
in this unit.
a unit of radio transmission rate equal to 1 million symbols
per second.
a common U.S. abbreviation for the metric ton or tonne
(1000 kilograms).
see metric ton unit, above.
mu or mou
a traditional unit of land area in China. The traditional mu is about 675
square meters or 800 square yards. However, the colonial customs authorities
used a larger mu equal to 8273.75 square feet, 919.3 square yards, or 768.65
square meters. In modern China, the mu is often reckoned to be exactly 1/15
hectare, which is 666.667 square meters or
797.327 square yards.
a traditional Dutch unit of volume for grains and other dry commodities.
Originally varying from market to market, the unit was declared equal to the
hectoliter (about 3.5315 cubic feet or 2.838 U.S. bushels)
when the metric system was introduced in the Netherlands. With this definition
it is still in use.
mug [1]
an informal contraction of “metric slug”. See TME or hyl.
mug [2]
another name for a slinch.
a traditional Scottish unit of liquid volume. The mutchkin is about 15 British
fluid ounces, which is about 426 milliliters
or almost exactly 0.9 U.S. pint.
abbreviation for meter of water equivalent, a unit used in nuclear physics
to describe the shielding around a reactor, accelerator, or detector. 1 mwe
of any material (such as rock, gravel, etc.) is a thickness of that material
providing shielding equivalent to one meter of water.
MWe, MWt
symbols used in the electric power industry to describe the size of generating
plants. MWe is the symbol for the actual output of a generating station in
megawatts of electricity; MWt is used for the heat energy, or thermal output,
required to operate the generators. Thermal output is typically about three
times the electric output.
Mya or mya
a common abbreviation (in English speaking countries) for “million years
ago.” The form “Mya” is recommended, since the capital M, taken from the metric
prefix mega- (M-), is the appropriate symbol for a million.
myria- (my-)
a metric prefix meaning 10 000. This prefix was part of the original metric
system of 1795 and was used throughout the nineteenth and early twentieth
centuries. It has been obsolete officially since 1960, when the CGPM adopted
the standard list of SI prefixes. The ancient Greek word myrios means
countless, without number. This was modified by later Greeks to form a word
myrioi meaning ten thousand. The word myriad, generally used
today to mean an indefinitely large number, originally meant the number 10
myriagram (myg)
a metric unit of mass equal to 10 000 grams or 10 kilograms (about 22.046
pounds). Although it is considered obsolete
now, the myriagram was a useful unit comparable to the English quarter
or Spanish arroba.
myriameter (mym)
an obsolete metric unit of distance equal to 10 000 meters or 10 kilometers
(about 6.2137 miles).
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